Temporal distribution of the annual nymphal stock of Ixodes ricinus ticks

The human risk of contracting Lyme disease or other tick borne diseases tra nsmitted by the tick species Ixodes ricinus is broadly linked to the tick n ymph density. The study was performed in Rambouillet forest (Yvelines, Fran ce), a known focus of Lyme borreliosis, from January 1997 to December 1999. We used a nymph sampling methodology which permitted us to obtain a monthl y nymph density index (from 0 to 5). Studying the seasonal nymph and larval activity patterns and estimating the larval developmental duration, we dem onstrate the existence of an annual nymphal stock. Secondly, we elucidate h ow this stock is distributed throughout the year, month by month. Its distr ibution is principally dependent on two factors: the monthly mean ambient t emperature and the proportion of active nymphs which find a host each month . Expected monthly nymph densities derived from a theoretical model describ ing the temperature-dependent stock distribution gave a good fit to the obs erved densities, accounting for between 76-86% of the monthly variation in observed nymph densities. Predicting the temporal distribution of nymph act ivity within a stable Lyme borreliosis focus enables more precise identific ation of risk periods.